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# Common Request Handler example.
This example shows a `Span` used with `RequestHandler`, which is used as a middleware (as in web frameworks) to manage a new `Span` per operation through its `before_request()`/`after_response()` methods.
Implementation details:
- For `threading`, no active `Span` is consumed as the tasks may be run concurrently on different threads, and an explicit `SpanContext` has to be saved to be used as parent.
- For `gevent` and `asyncio`, as no automatic `Span` propagation is done, an explicit `Span` has to be saved to be used as parent (observe an instrumentation library could help to do that implicitly - we stick to the simplest case, though).
- For `tornado` and `contextvars`, as parent `Span` propagates automatically (even if the tasks are called through different coroutines), we **do** leverage the active `Span`.
RequestHandler implementation:
```python
def before_request(self, request, request_context):
# If we should ignore the active Span, use any passed SpanContext
# as the parent. Else, use the active one.
if self.ignore_active_span:
# Used by threading, gevent and asyncio.
span = self.tracer.start_span('send',
child_of=self.context,
ignore_active_span=True)
else:
# Used by tornado and contextvars.
span = self.tracer.start_span('send')
```
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